The use of a supercharger to increase or “boost” the air pressure in an intake manifold of an internal combustion engine to result in an engine having a greater horsepower output capability is known. To pressurize the air, a supercharger must spin rapidly—generally more rapidly than the engine itself. Typically this is accomplished by providing a crankshaft or drive pulley that is larger than the supercharger or driven pulley.
An accessory belt can be used to, among other things, drive a positive displacement supercharger that rotates at a fixed ratio with respect to engine crankshaft rpm. A supercharger pulley rotates at speeds that are generally physically defined by the ratio between the supercharger pulley and an engine crankshaft pulley. As such, supercharger pulley revolutions per minute (rpm) is defined by an engine rpm pulley ratio. Generally, the maximum rpm of a supercharger must not be exceeded throughout the entire engine rpm range. Moreover, the maximum value is commonly used to define the largest ratio that can be achieved with a fixed ratio pulley size, which can limit the amount of airflow that can be provided in a lower rpm range of an engine.
Generally, supercharger volume airflow is based upon: supercharger displacement (volume per revolution), volumetric efficiency (percent of displaced volume filled every revolution), and rotor rpm. However, for some applications, a two-speed drive system can be used to provide a different supercharger-to-engine rpm ratio for use in the lower rpm range of an engine. Among other things, a two-speed system permits two supercharger rotor speed selections for each supercharger pulley rpm value. Such a system can provide, inter alia, more airflow and more engine torque without requiring a supercharger constructed for a larger displacement. Further, for some applications, a two-speed drive system can provide improved performance in a lower rpm range, while maintaining a standard performance in the higher rpm range, and while still not exceeding maximum supercharger rpm. Moreover, with two different ratios available, a supercharger can provide gear ratios and supercharger shift points that may be better optimized for a particular engine design to provide desirable power characteristics.